The field of wireless power transmission for low and medium power levels is currently focused on resonant technologies. A controlled-frequency circuit is used to generate an AC current through a magnetic coil at a specific frequency, which is used as an energy transmitter. The flux created in the transmission coil excites magnetics in a receiver circuit due to electromagnetic induction. A receiver is based on a tuned LC circuit, which resonates at the specific transmission frequency. This tuned LC circuit reacts to the current induced in the receiver coil, amplifying the resultant AC voltage due to resonance of the LC circuit. The AC voltage created across the LC circuit is then generally rectified, the resultant DC voltage being used by the remote electronics fed by the resonant wireless power receiver.
An appropriately designed receiver uses a high-quality factor (Q) LC circuit that is perfectly tuned to the transmission frequency, maximizing the voltage available to the receiver-fed electronics. As the tuned receiver LC circuit is operating at resonance, however, the resultant voltage could theoretically climb to astronomical levels, potentially damaging the electronics fed by the receiver. A method is therefore required to regulate the resultant receiver voltage to levels that are controlled and useable by the associated circuitry.
It is desirable to control the amplitude of a resonant receiver directly, resulting in a post-rectified DC voltage regulated to a level that is appropriate for an associated electronic system. Two methods of receiver amplitude control commonly discussed are (1) selective re-tuning of the LC circuit resonant frequency, or (2) Q reduction of the LC circuit. In the first case, the resonant frequency of the receiver circuit is shifted away from the transmission frequency, such that the gain characteristic is reduced at the transmission frequency such that the desired resultant voltage is achieved. In the second case, the tuned frequency remains the same, so the peak gain of the tuned receiver remains at the transmission frequency, but resistance is added to the resonant circuit reducing the Q of the circuit, and thus reducing the gain, such that the desired resultant voltage is achieved.
There are several problems with these two methods of resonant gain-adjustment, particularly when examining appropriateness for use in an automatic regulation system. Variation of resonant frequency, or re-tuning the circuit, is the most desirable regulation method since a high-Q LC circuit is extremely efficient, as it theoretically dissipates no power. Unfortunately, electronically controlled variable capacitors or electronically variable inductors are objects of fantasy, and thus the adjustment of resonant frequency characteristics of an LC circuit through dynamic variation of the effective inductance or capacitance is not a directly practical approach for regulation circuits. Q-reduction is a practical solution, as incorporation of electronically variable resistance is trivial, but Q-reduction through additional resistance necessarily means additional power dissipation, and the subsequent loss of efficiency quickly becomes problematic.
It would be desirable to develop system and methodology for providing voltage regulation of a wireless power receiver by re-tuning the effective gain characteristic of a resonant LC circuit without incorporation of additional passive components. Additionally, it would be desirable to control a wireless power receiver without direct introduction of resistive components so as to minimize resultant parasitic power dissipation.